Bohemian waxwing

Bohemian waxwing
The Bohemian waxwing (Bombycilla garrulus) is a starling-sized passerine bird that breeds in the northern forests of the Palearctic and North America. It has mainly buff-grey plumage, black face markings and a pointed crest. Its wings are patterned with white and bright yellow, and some feather tips have the red waxy appearance that give this species its English name. The three subspecies show only minor differences in appearance. Females are similar to males, although young birds are less well-marked and have few or no waxy wingtips. Although the Bohemian waxwing's range overlaps those of the cedar and Japanese waxwings, it is easily distinguished from them by size and plumage differences.
The breeding habitat is coniferous forests, usually near water. The pair build a lined cup-shaped nest in a tree or bush, often close to the trunk. The clutch of 3–7 eggs is incubated by the female alone for 13–14 days to hatching. The chicks are altricial and naked, and are fed by both parents, initially mostly with insects, but thereafter mainly fruit. They fledge about 14–16 days after leaving the egg. Many birds desert their nesting range in winter and migrate farther south. In some years, large numbers of Bohemian waxwings irrupt well beyond their normal winter range in search of the fruit that makes up most of their diet.
Waxwings can be very tame in winter, entering towns and gardens in search of food, rowan berries being a particular favourite. They can metabolise alcohol produced in fermenting fruit, but can still become intoxicated, sometimes fatally. Other hazards include predation by birds of prey, infestation by parasites and collisions with cars or windows. The Bohemian waxwing's high numbers and very large breeding area mean that it is classified as being of least concern by the International Union for Conservation of Nature.
Taxonomy
The waxwings are a family, Bombycillidae, of short-tailed stocky birds with soft plumage, a head crest and distinctively patterned wings and tails. There are three species, the Bohemian, cedar, and Japanese waxwings. DNA studies and shared features such as a relatively large size, grey underparts and similar undertail patterns suggest that the Japanese and Bohemian waxwings are most closely related within the genus. Although only the cedar and Bohemian waxwings normally have red tips on their wing feathers, this feature is occasionally shown by the Japanese waxwing, suggesting that this was originally a whole-family characteristic that has been lost in one species, rather than an indicator of a close relationship. DNA analysis confirms that the cedar waxwing diverged early from the other members of the family. Outside the genus, the closest relatives of the waxwings are believed to be the silky-flycatchers, the palmchat, and the grey hypocolius, all of which have sometimes been included in the Bombycillidae.
The Bohemian waxwing was described by Carl Linnaeus in his 1758 10th edition of Systema Naturae as Lanius Garrulus. The waxwings were moved to their own genus, Bombycilla, by Louis Jean Pierre Vieillot in 1808. The genus name Bombycilla comes from the Greek bombux, "silk" and the Modern Latin cilla, "tail"; this is a direct translation of the German Seidenschwanz, "silk-tail", and refers to the silky-soft plumage of the bird. The species name garrulus is the Latin for talkative and was applied to this bird, as "Garrulus Bohemicus", by Conrad Gessner in 1555; the term is a reference to a supposed likeness to the Eurasian jay (Garrulus glandarius) rather than to the waxwing's vocalisations. The English name "waxwing" refers to the bright red tips of the secondary feathers on its wings, which look like drops of sealing wax, while "Bohemian" follows Gessner's usage, and may refer to the Romani, alluding to the bird's wanderings, or to its presumed origin from Bohemia. "Waxwing" and "Bohemian waxwing" were first recorded in 1817, the former as a reference to Vieillot's separation of this bird from the "chatterers".
There are three recognised subspecies:
B. g. garrulus (Linnaeus, 1758): the nominate subspecies, breeds in northern Europe from northern Norway east to the Ural Mountains.
B. g. centralasiae (Poliakov, 1915): breeds from the Urals eastwards across northern Asia.
B. g. pallidiceps (Reichenow, 1908): breeds in northwestern North America.
The differences between these forms are small and clinal, and the species could be possibly considered as monotypic. The fossil record includes Pleistocene deposits from the UK and the Carpathian Mountains.
Description
The Bohemian waxwing is a starling-sized bird in length with a wingspan, and an average weight of . It is short-tailed, mainly brownish-grey, and has a conspicuous crest on its head. The male of the nominate subspecies has a black mask through the eye and a black throat. There is a white streak behind the bill and a white curve below the eye. The lower belly is a rich chestnut colour and there are cinnamon-coloured areas around the mask. The rump is grey and the tail ends in a bright yellow band with a broad black border above it. The wings are very distinctive; the flight feathers are black and the primaries have markings that produce a yellow stripe and white "fishhooks" on the closed wing. The adult's secondaries end in long red appendages with the sealing wax appearance that gives the bird its English name. The eyes are dark brown, the bill is mainly black, and the legs are dark grey or black. In flight, the waxwing's large flocks, long wings and short tail give some resemblance to the common starling, and its flight is similarly fast and direct. It clambers easily through bushes and trees but only shuffles on the ground.
The soft, dense feathers are kept in good condition by preening. The red waxy tips are the extended and flattened ends of feather shafts, pigmented with astaxanthin and enclosed in a transparent sheath. A study of the cedar waxwings showed that the red tips are few or absent until the third year of life for that related species. All adult waxwings have a complete moult annually between August and January. Juveniles moult at the same time but retain their flight and some other wing feathers.
The female Bohemian waxwing is very similar to the male, but has a narrower yellow terminal band to the tail, a less defined lower edge to the black throat and slightly less distinctive wing markings. Juveniles are duller than adults, with whiter underparts, only a few red wing tips, no black on the throat and a smaller black face mask. Compared to the nominate subspecies, eastern B. g. centralasiae is paler, greyer and has little reddish-brown behind the bill. The American subspecies B. g. pallidiceps has more colouring on the cheeks and forehead than the nominate form and is otherwise generally duller in appearance.
The range of the Bohemian waxwing overlaps those of both the other members of the genus. The cedar waxwing is smaller than the Bohemian; it has browner upperparts, a white undertail and a white line above the black eye patch. Adult cedar waxwings have a yellowish belly, and all ages have less strongly patterned wings than the Bohemian waxwing. The Japanese waxwing is easily distinguished from its relatives; it has a red terminal band to the tail, the black mask extends up the rear of the crest, and there is no yellow stripe or red tips on the wings.
The Bohemian waxwing's call is a high trill sirrrr. It is less wavering and lower-pitched than that of the cedar waxwing, and longer and lower-pitched than the call of the Japanese waxwing. Other calls are just variants of the main vocalisation; a quieter version is used by chicks to call parents, and courtship calls, also given during nest construction, have a particularly large frequency range. Although not a call as such, when a flock takes off or lands, the wings make a distinctive rattling sound that can be heard away.
Distribution and habitat
The Bohemian waxwing has a circumpolar distribution, breeding in northern regions of Eurasia and North America. In Eurasia, its northern nesting limit is just short of the treeline, roughly at the 10 °C July isotherm, and it breeds south locally to about 51°N. Most birds breed between 60–67°N, reaching 70°N in Scandinavia. The North American subspecies breeds in the northwestern and north central areas of the continent, its range extending southwards beyond the US border in the Rocky Mountains.
This waxwing is migratory with much of the breeding range abandoned as the birds move south for the winter. Migration starts in September in the north of the range, a month or so later farther south. Eurasian birds normally winter from eastern Britain through northern parts of western and central Europe, Ukraine, Kazakhstan and northern China to Japan. North American breeders have a more southeasterly trend, many birds wintering in southeast Canada, with smaller numbers in the north central and northeastern US states. Birds do not usually return to the same wintering sites in successive years. One bird wintering in the Ukraine was found to the east in Siberia in the following year.
In some years, this waxwing irrupts south of its normal wintering areas, sometimes in huge numbers. The fruit on which the birds depend in winter varies in abundance from year to year, and in poor years, particularly those following a good crop the previous year, the flocks move farther south until they reach adequate supplies. They will stay until the food runs out and move on again. In what may be the largest ever irruption in Europe, in the winter of 2004–2005, more than half a million waxwings were recorded in Germany alone. This invasion followed an unusually warm, dry breeding season. In 1908, an American flock wide was noted as taking two to three minutes to fly over.
The breeding habitat is mature conifers, often spruce although other conifers and broadleaf trees may also be present. More open, wet areas such as lakes and peat swamps with dead and drowned trees are used for feeding on insects. Lowlands, valleys and uplands are used in Eurasia, although mountains tend to be avoided. However, the North American subspecies nests in Canada at altitudes between . Outside the breeding season, the waxwing will occupy a wide range of habitats as long as suitable fruits such as rowan are available. It may be found by roads, in parks and gardens or along hedges or woodlands edges. It shows little fear of humans at this time. In winter, waxwings roost communally in dense trees or hedges, sometimes with American robins, fieldfares or other wintering species.
Behaviour
Breeding
Bohemian waxwings start their return from the wintering grounds in February or March, but northern breeders do not reach their breeding areas until April or early May. This monogamous species nests mainly from mid-June to July.
Waxwings are not highly territorial, and, although normally solitary breeders, several pairs may nest close together where there are a number of good nest sites. Males may sometimes deter rivals from approaching their mates, and females may squabble over nest sites. Aggression is shown by sleeking down the feathers and crest, showing the black throat, and opening the bill. The breeding display is almost the opposite of this; the male erects his body and crest feathers, and turns his head away from the female. The male may repeatedly present a gift of a small item, often food, to his partner, placing it in her open bill. In about 90% of cases, this display does not lead to copulation. Older males have more red tips to the wings and are preferred by females.
The nest, built by both sexes, is a cup of thin twigs lined with softer material such as fine grass, moss, fur or lichen. It is constructed above the ground in a pine or scrub, commonly close to the trunk. The eggs are a glossy pale blue spotted with black and grey and the clutch is 3–7 eggs, although five or six is most common. The average size of the egg is , and it weighs , of which 5% is shell. The eggs are incubated for 13–14 days by the female alone. She is fed regurgitated berries by her mate, and rarely leaves the nest. The chicks are altricial and naked, and have bright red mouths; they are fed by both parents, although the male brings most of the food, mainly insects, in the first few days. The young are subsequently fed largely with fruit. The chicks fledge about 14–16 days after hatching. They are fed by the adults for about two weeks after fledging.
Breeding densities of this waxwing are typically low compared to other passerines, usually less than ten birds per square kilometre (26 per square mile) even in good habitat, although up to 35·6 birds per square kilometre (92 per square mile) have been found in Russia. One brood each year is normal. Maximum recorded ages are 5 years 10 months in North America and more than 13 years 6 months in Europe. The average life expectancy is unknown. Significant causes of death include predation, collisions with windows and cars, and poisoning by road salt consumed when drinking.
Feeding
Waxwings are primarily fruit eaters, but also consume insects during the breeding season. Mosquitoes and midges are the most common prey, but many other insects and some spiders are eaten. They are caught mainly by flycatching from an open perch, but some may be picked off vegetation. Fruit is normally picked from trees, sometimes from the ground, and is usually swallowed whole. In the summer, Vaccinium and Rubus species and Canada buffaloberry are important items of their diet, while cotoneaster, juniper, haws, rose hips and apples predominate outside the breeding season. Rowan berries are a favourite food, and are eaten whenever available.
Waxwings can eat huge numbers of berries, each bird sometimes consuming several hundred a day, more than double its own weight. One individual was recorded as eating between 600 and 1,000 cotoneaster berries in six hours, and defecating every four minutes. Waxwings travel significant distances when feeding and help to disperse the fruit seeds. Waxwings forage in large flocks, sometimes of several hundred birds, which enables them to overwhelm birds such as mistle thrushes attempting to defend their fruit trees.
Fruit is rich in sugar but deficient in other nutrients, so it must be eaten in large quantities. Bohemian waxwings have a large liver which helps to convert sugar to energy. They can metabolise ethanol produced from the fermentation of those sugary fruits more efficiently than humans, but may still become intoxicated, sometimes fatally. Waxwings often drink water or eat snow in winter, since the sugar in their fruit diet tends to dehydrate the birds through an osmotic effect. In the summer, the fruits are juicier and water is less of a problem.
In the past, the arrival of waxwings sometimes coincided with epidemics of cholera or plague, and led to the old Dutch and Flemish name Pestvogel, "plague bird". The juniper berries on which they fed were thought to offer protection, and people consumed the fruit and burned branches to fumigate their houses.
Predators and parasites
Waxwings are preyed upon by birds of prey including rough-legged buzzards, Eurasian sparrowhawks, prairie falcons, and great grey shrikes. Merlins attack winter flocks, including those in cities. When alarmed, Bohemian waxwings "freeze" with bill and neck pointing upwards. If this fails, they fly, calling noisily.
Bohemian waxwings are not brood parasitised by the common cuckoo or its relatives in Eurasia because the cuckoo's young cannot survive on a largely fruit diet. In North America, the waxwing's breeding range has little overlap with brown-headed cowbird, another parasitic species. Nevertheless, eggs of other birds placed in a Bohemian waxwing's nest are always rejected. This suggests that in the past, perhaps 3 million years ago, the ancestral waxwing was a host of a brood parasitic species, and retains the rejection behaviour acquired then.
Parasitic mites include Syringophiloidus bombycillae, first identified on this species, and the nasal mite Ptilonyssus bombycillae. Blood parasites include Trypanosoma species, and a Leucocytozoon first identified in this waxwing. Bohemian waxwings may carry flatworms and tapeworms, but levels of parasitic worm infestation are generally low.
Status
The global population of the Bohemian waxwing has been estimated at more than three million birds, and the breeding range covers about 12.8 million km2 (4.9 million mi2). Although this species' population, as of 2013, appears to be declining, the decrease is not rapid nor large enough to trigger conservation vulnerability criteria. Given its high numbers and huge breeding area, this waxwing is therefore classified by the International Union for Conservation of Nature as being of least concern. The woodlands used by this species are well to the north of major human populations, and the birds can use disturbed habitats, so there are no serious long-term threats to this species.
Gallery
Notes
References
Cited texts
Rausch, Robert L. "Biology of Avian Parasites: Helminthes" in
External links
Bohemian waxwing Species Account – Cornell Lab of Ornithology
Bohemian waxwing
Holarctic birds
Bohemian waxwing
Taxa named by Carl Linnaeus